1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal device and, more specifically, to structures of pixel portions and non-pixel portions. The present invention further relates to a method of manufacturing such a ferroelectric liquid crystal device.
2. Description of the Background Art
FIG. 23 is a perspective view showing schematically a conventional ferroelectric liquid crystal device. Transparent electrodes 3a are arranged spaced apart from each other on a main surface of a transparent plate 2a. Transparent electrodes 3b are arranged spaced apart from each other on a main surface of a transparent plate 2b. The transparent plates 2a and 2b are positioned opposing to each other to form a space. The transparent electrodes 3a and the transparent electrodes 3b intersect with each other in three dimensions. Areas at which the transparent electrodes 3a and 3b intersect with each other constitute pixels portions. The remaining portions are the non-pixel portions. Display patterns are formed by combining the pixel portions. Ferroelectric liquid crystals are sealed
in the space formed by the transparent plates 2a and 2b The ferroelectric liquid crystals are not shown in FIG. 23.
The conventional ferroelectric liquid crystal device 1 will be further described with reference to FIGS. 24 and 25. FIG. 24 is a cross sectional view taken in a direction of an arrow XXIV of FIG. 23. FIG. 25 is a cross sectional view taken in a direction of an arrow XXV of FIG. 24.
As shown in FIG. 24, the ferroelectric liquid crystal device 1 has a structure wherein liquid crystal molecules 6 are sandwiched by the transparent plate 2a, on which transparent electrodes 3a are formed, and a transparent plate 2b, on which transparent electrodes 3b are formed. The portion represented by 7a shows that the left end portion of the liquid crystal molecule 6 is projected upward from the surface of the sheet. The portion represented by 7b shows that the right end portion of the liquid crystal molecule 6 is projected upward from the surface of the sheet. The reference character 8 indicates spontaneous polarization of the liquid crystal molecule 6. The head of the arrow corresponds to the positive polarity, while the tail of the arrow corresponds to the negative polarity. The reference character 9 shows a line indicating a boundary between a layer of the liquid crystal molecule 6 and a layer of the liquid crystal molecule 6. The line represented by 9 will be hereinafter called as a boundary.
An insulating film 4a is formed on the transparent electrode 3a. An alignment film 5a is formed on the insulating film 4a. An insulating film 4b is formed on the transparent electrode 3b. An alignment film 5b is formed on the insulating film 4b. The insulating films 4a, 4b and alignment films 5a and 5b are not shown in FIG. 23.
The alignment films 5a and 5b serve to make the orientation of the liquid crystal molecules 6 parallel to the main surface of the transparent plates 2a and 2b. However, the boundaries 9 are not always made parallel to each other dependent on the alignment films 5a and 5b. Rubbing is done on the transparent plates 2a and 2b, so that the boundaries 9 are made parallel to each other.
FIG. 26 shows rubbing on the transparent plate 2a on which the alignment film is formed. A rubbing cloth 11 is attached around a rubbing roller 12. The rubbing roller 12 is rotated in the direction of the arrow, and the transparent plate 2a is fed in contact with the rubbing cloth 11. Consequently, a plurality of linear grooves are formed on the transparent plate 2a. These grooves are called rubbings. The boundaries 9 are aligned with these grooves. The intensity of polarity of the alignment film is considered to be changed by the rubbings. Rubbing is also done on the transparent plate 2b on which the alignment film is formed.
The ferroelectric liquid crystal device has the following characteristics. Namely, bistability, memory effect and high responsiveness. Bistability will be described at first.
As shown in FIG. 25, the liquid crystal molecule 6 is stable when the longitudinal axis thereof is inclined by .theta. from the normal 10 of the boundary 9 and when it is inclined by -.theta.. Namely, the liquid crystal molecule 6 is in a bistable state.
How the liquid crystal molecule 6 is set in a bistable state will be described with reference to FIGS. 24, 25, 27 and 28. In FIG. 27, only the orientation of the liquid crystal molecules 6 is different from that in the ferroelectric liquid crystal device 1 shown in FIG. 24. FIG. 28 is a cross sectional view taken along the direction of an arrow XXVIII of FIG. 27. As shown in FIGS. 27 and 28, if the polarity of the alignment film 5a is different from that of the alignment film 5b, for example if the polarity of the alignment film 5a is negative and that of the alignment film 5b is positive, the liquid crystal molecules 6 will be as shown in FIGS. 27 and 28. This is because the liquid crystal molecules 6 have spontaneous polarization 8. As shown in FIG. 28, the liquid crystal molecules 6 are stable with the longitudinal axes being inclined by .theta. from the normal 10. When a positive voltage is applied to the transparent electrode 3a and a negative voltage is applied to the transparent electrode 3b, the direction of the spontaneous polarization 8 of those liquid crystal molecules 6 positioned between the transparent electrodes 3a and 3b is inverted as shown in FIG. 24. Referring to FIG. 25, the liquid crystal molecules 6 on the transparent electrode 3b have their longitudinal axes inclined by -.theta. from the normal 10. However, when the application of the voltages is stopped, the state shown in FIGS. 27 and 28 are regained. The reason for this is that the alignment film 5a has negative polarity and the alignment film 5b has positive polarity. Therefore, the liquid crystal molecules 6 are not stable when the longitudinal axes are inclined by -.theta. from the normal 10. In that case, the bistable state is not realized.
If the polarity of the alignment film 5a and the polarity of the alignment film 5b are made the same and the intensities of the polarity are made approximately the same, then stable state as shown in FIG. 24 and 25 is realized even when a positive voltage is applied to the transparent electrode 3a and a negative voltages is applied to the transparent electrode 3b. Thereafter, the application of voltages is stopped, as shown in FIGS. 24 and 25. In that case, even if the polarities of the alignment film 5a and of the alignment film 5b are the same, the liquid crystal molecules 6 are influenced by the stronger polarity after the application of voltages is stopped, when the intensity of polarity is different. Then, a negative voltage is applied to the transparent electrode 3a shown in FIG. 24 and a positive voltage is applied to the transparent electrode 3b. Then the liquid crystal molecules 6 will be at the state shown in FIGS. 27 and 28. Even when the application of voltages is stopped, the state of the liquid crystal molecules 6 shown in FIGS. 27 and 28 is maintained.
Namely, the bistable state of the liquid crystal molecules 6 can be realized by making the polarity of the alignment film 5a the same as that of the alignment film 5b and by making the intensities of polarities approximately equal to each other.
The ferroelectric liquid crystal device 1 has memory effect, since the orientation of the liquid crystal molecules 6 is maintained even after the application of voltages is stopped. In addition, since the liquid crystal molecule 6 has both positive and negative polarities called spontaneous polarization, the orientation of the liquid crystal molecule 6 is immediately changed when the voltage is applied thereto. Namely, the ferroelectric liquid crystal device 1 has high responsiveness.
The operation of the ferroelectric liquid crystal device 1 will be described with reference to FIGS. 24, 25, 27 and 28.
Referring to FIG. 24, when a positive voltage is applied to the transparent electrode 3a and a negative voltage is applied to the transparent electrode 3b, the liquid crystal molecules 6 are arranged as shown in FIGS. 24 and 25.
Thereafter, at the state of FIGS. 24 and 25, a negative voltage is applied to the transparent electrode 3a and a positive voltage is applied to the transparent electrode 3b. Then, orientation of those liquid crystal molecules 6 which are positioned between the transparent electrode 3a and the transparent electrode 3b is changed as shown In FIGS. 27 and 28. The liquid crystal molecules 6 positioned on the non-electrode portion 18 are not influenced by the voltages, so that the orientation thereof is not changed.
A deflecter transmitting light only in the direction shown by the arrow D of FIG. 25 is attached to the transparent plate 2a shown in FIG. 24, and a deflecter transmitting light only in the direction shown by the arrow E of FIG. 25 is attached below the transparent plate 2b. When light is projected from above, an area where the transparent electrode 3a intersect with the transparent electrode 3b in three dimensions is set to a dark state, while an area where the non-electrode portion 18 is positioned is set to a bright state. The reason for this will be described with reference to FIG. 25. Since the angle of light deflected in the direction D is the same as the angle of the liquid crystal molecules 6 between the transparent electrodes 3a and 3b, the light directly passes through the liquid crystal molecules 6. The light deflected in the direction D cannot pass through the deflector transmitting light only in the E direction, so that the area where the transparent electrodes 3a and 3b intersect three dimensionally is set to a dark state. Meanwhile, the angle of light deflected in the direction D is different from the angle of the liquid crystal molecules 6 positioned on the non-electrode portion 18. Therefore, the light is elliptically deflected when it passes through the liquid crystal molecules 6. Part of the elliptically deflected light passes through the deflector transmitting light only in the direction E, so that the non-electrode portion 18 is set to the bright state.
A method of driving the ferroelectric liquid crystal device 1 will be described in the following. FIG. 29 is a partial plan view showing pixel portions and non-pixel portions of the ferroelectric liquid crystal device. Electrodes 19a and 19b correspond to the transparent electrodes 3a shown in FIG. 23, while electrodes 21a and 21b correspond to transparent electrodes 3b shown in FIG. 23. Areas where the electrodes 19a, 19b and 21a and 21b intersect with each other are the pixel portions 23a, 23b, 23c and 23d, respectively. The remaining areas are the non-pixel portions 25.
A pulse such as shown by 27a of FIG. 30 is applied to the electrode 19a. A pulse such as shown by 27b of FIG. 30 is applied to the electrode 19b. A pulse such as shown by 27c of FIG. 30 is applied to the electrode 21a. Then, a voltage having the pulse of 27d, that is, the voltage of the pulse 27a minus the voltage of the pulse 27c, is applied to the pixel portion 23a, as shown in FIG. 30. The pulse 27d exceeds the threshold value 29. Therefore, the orientation of the liquid crystal molecules arranged in the pixel portion 23a is changed.
A voltage of the pulse 27e, that is, the voltage of the pulse 27b minus the voltage of the pulse 27c, is applied to the pixel portion 23b. The pulse 27e does not exceed the threshold value 29. Therefore, the liquid crystal molecules in the pixel portion 23b are kept as they are.
As shown in FIG. 30, the pulse 27d applied to the pixel portion 23a does not always exceed the threshold value. Such a driving method is called multiplexing driving.
The conventional ferroelectric liquid crystal device has the following two drawbacks. The first of these will be described with reference to FIGS. 31, 32 and 33. FIG. 31 is a vertical sectional view of a conventional ferroelectric liquid crystal device. FIG. 32 is a cross section of the ferroelectric liquid crystal device shown in FIG. 31 taken from the direction of the arrow XXXII. FIG. 33 is a partial plan view of the display surface of the ferroelectric liquid crystal device shown in FIG. 31. The reference characters in FIGS. 31 and 32 denote the same portions denoted by the same reference characters of FIGS. 24 and 25.
As shown in FIG. 31, the distance between the alignment films 5a and 5b positioned on the non-electrode portion 18 (portion where there is no electrode) is longer than that between the alignment films 5a and 5b on the transparent electrodes 3a and 3b, since the transparent electrodes 3a and 3b are not formed on the non-electrode portion 18. Therefore, the difference of intensity of the polarities between the alignment films 5a and 5b positioned on the non-electrode portion 18 is further reduced from the difference of intensity of polarities between the alignment films 5a and 5b on the transparent electrodes 3a and 3b. Consequently, as shown in FIGS. 31 and 32, when liquid crystal is sealed, liquid crystal molecules 6 inclined by .theta. from the normal 10 and the molecules inclined by -.theta. from the normal 10 may possibly be mixed on the non-electrode portion 18.
When such a ferroelectric liquid crystal device is used, there are portions 35a transmitting light and portions 35b not transmitting light are mixed on the non-pixel portion 31 which is the non-electrode portion, as shown in FIGS. 33. Since there are portions 35a transmitting light and portions 35b not transmitting light mixed, the state of display on the non-pixel portion 31 will be uneven. The reference characters 33 represents pixel portions.
The second drawback of the conventional ferroelectric liquid crystal device will be described with reference to FIGS. 34, 35 and 36. The same reference characters in FIGS. 34 and 35 represent the same portions as represented in FIG. 29.
Referring to FIG. 34, the liquid crystal molecules in the pixel portions 23a and 23c are of the same orientation as the liquid crystal molecules 6 on the non-electrode portion 18 shown in FIG. 25. The liquid crystal molecules in the pixel portions 23b and 23d are of the same orientation as the liquid crystal molecules 6 on the electrode portion 3b shown in FIG. 25. Even if the voltage applied to the pixel portions 23a, 23b, 23c and 23d does not exceed the threshold value, regions 37 in which spontaneous polarization of the liquid crystal molecules is inverted are generated after a time lapse of a prescribed period, as shown in FIG. 35. The reason for this may be the fact that the pulse 27d or the pulse 27e shown in FIG. 30 is always applied to the pixel portions 23a, 23b, 23c and 23d.
Inversion of the spontaneous polarization of the liquid crystal molecules spreads in the direction shown by the arrow A in FIG. 35. The reason will be described. FIG. 36 is a perspective view showing arrangement of liquid crystal layers in the pixel portion 23a. As shown in FIG. 36, a large number of liquid crystal layers 39 having lateral "V" shape are arranged in the pixel portion 23a. The liquid crystal layers 39 are arranged in the same direction. Since inversion of the spontaneous polarization starts from the liquid crystal molecule in the liquid crystal layer 39 near the side shown by the letter B, the inversion of the spontaneous polarization proceeds from B to C. The direction from B to C is the same as the direction shown by the arrow A in FIG. 35.
A ferroelectric liquid crystal device in which the state of display of the pixel portions can be made stable is disclosed in Japanese Patent Laying-Open No. 1-179915. The ferroelectric liquid crystal device shown in the Patent Laying-Open No. 1-179915 will be described with reference to FIGS. 37, 38, 39 and 40.
As shown in FIG. 37, a plurality of electrodes 43 are arranged spaced apart from each other on a main surface of a plate 41. A molybdenum film 45 is formed on one side edge of each of the electrode 43. Two of such plates 41 are prepared for manufacturing the ferroelectric liquid crystal device.
FIGS. 39 and 40 are plan views of a plate used for manufacturing the ferroelectric liquid crystal device. As shown in FIG. 39, electrodes 43a are arranged spaced apart from each other on the main surface of the plate 41a. Molybdenum films 45a are formed on one side edge of each of the electrodes 43a. As shown in FIG. 40, electrodes 43b are arranged spaced apart from each other on the plate 41b. The direction of extension of the electrode 43b is orthogonal to the direction of extension of the electrode 43a shown in FIG. 39. Molybdenum films 45b are formed on one side edge of the electrodes 43b. The plate 41b shown in FIG. 40 is arranged to be opposed to the plate 41a shown in FIG. 39. The edge portion represented by G of the plate 41b faces the edge portion represented by D of the plate 41a, while the edge portion represented by F of the plate 41b faces the edge portion represented by E of the plate 41a. FIG. 38 is a plan view of a pixel portion of the ferroelectric liquid crystal device. The region represented by B in FIG. 36 is on the right side of the pixel 47, and the region C of FIG. 36 is on the left side of the pixel portion 47. Therefore, recess of the lateral "V" shape of the liquid crystal layer 39 shown in FIG. 36 faces the side of the molybdenum film 45b.
If the pixel portion 47 is structured in this manner, the liquid crystal molecules in the pixel portion 47 is not inverted unless the voltage applied to the pixel portion 47 exceeds the threshold value. The reason for this may be the fact that there is a molybdenum film 45b on the side B where the liquid crystal molecules tend to be inverted.
However, the unevenness of display of the non-pixel portion, which was the first drawback, could not be eliminated by the ferroelectric liquid crystal device disclosed in Japanese Patent Laying-Open No. 1-179915.
In addition, as shown in FIG. 37, the distance between the electrodes 43 is small, so that there is a possibility of short circuit between electrodes 43, as the molybdenum film 45 may possibly be in contact with the adjacent electrode 43.
The present invention was made to solve the above described problem of the prior art.